This application claims priority to European Application Serial Number 00870159.1 filed on Jul. 10, 2000, the disclosure of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention is related to a plaster retarding composition.
2. Description of the Related Art
The setting of plaster of Paris is known for a long time. Gypsum plaster sets quickly due to its natural process of crystallisation.
This crystallisation of gypsum (CaSO4.2H2O) from a suspension of Ca sulfate hemihydrate is known as the setting of plaster. For commercial applications, the retardation of plaster to a desired level is required.
In the 1960""s it was reported that tartaric acid and calcium tartrate had negligible retarding action in the setting of plasters. Potassium sulfate was also reported to accelerate the setting (Combe, E. C., Chemical Abstracts, Vol. 63, p. 12855).
Later, when the retarding action of gypsum plaster retarders of various chemical compositions was studied in relation to the pH value of the plaster, it was shown that tartaric acid and tartrates were capable of fully developing their retarding action only under strongly alkaline pH (Mallon, T., ZKG Int., Ed. B, Vol. 41(6), pp. 309-311 (1988)). The effects of retards (citric acid, malic acid, and tartaric acid) on the setting behaviour of plaster were studied to produce plasters with special properties which met site requirements. There it was shown that the retarders need varying amounts of Ca ions to be effective. The gypsum-tartaric acid system required additional Ca(OH)2. Tartaric acid xe2x80x94Ca(OH)2 had the largest effect retarding effect on plaster of Paris. Significant changes in crystal development were observed, the strength was halved, and setting retardation was significant. (Forg, G., ZKG Int., Ed. B, Vol. 42(5), pp. 229-32 (1989)).
It has been known for a long time that the particle size of the gypsum is of importance to the setting characteristics of plaster and to the mechanical properties of the hardened product (U.S. Pat. No. 3,652,309 and DE 3048506). It is also known that calcium tartrate may be obtained from various distillery wastes (vinasse) (FR 2109317 and EP 0919535A1), or by other processes as described in the documents GB 1183449 and U.S. Pat. No. 3,957,579.
The present invention aims to provide an improved retarding compositions for plaster that does not present the drawbacks of the state of the art, especially a product that can be easily manipulated and used by construction hands on building sites, such as plasterers, etc. A further aim of the present invention is to provide such a composition which has improved retarding characteristics for plaster and other related products.
Although the use of calcium tartrate as a retarder in the setting of plaster products is already described, the inventors have surprisingly found that reducing the particle size (granulometry) of a calcium tartrate polydispersed composition improves unexpectedly the retarding plaster setting. This means that a given amount of calcium tartrate with the reduced particle size will retard the setting of plaster more efficiently than the same amount of calcium tartrate with a non reduced (higher) particle size.
Therefore, a first aspect of the present invention is related to a preferably uncoloured polydispersed plaster retarding composition having a mean particle size lower than 30 xcexcm, preferably lower than 25 xcexcm, more preferably lower than 20 xcexcm, advantageously lower than 18 xcexcm or lower than 15 xcexcm.
Advantageously, said composition comprises less than 5%, more preferably less than 1%, more preferably less than 0.1%, of particles having a particle size higher than 40 xcexcm (the total of the particles being 100%).
The present invention is also related to the preparation process of said calcium tartrate composition wherein particles of calcium tartrate obtained after crystallisation from products resulting from distillery waste (vinasse) or from other biological processes, are purified and submitted to a reduction of particle size by various classical means such as milling, grinding in a mortar by impact in pneumatic dryers or other methods well known by the person skilled in the art. Preferably, the calcium tartrate is obtained according to the following steps:
cis-epoxysuccinate is obtained by catalytic epoxidation of maleic acid (preferably by using the method and the enzyme described in the document WO 99/21972),
the cis-epoxysuccinate is thereafter submitted to an enzymatic action of an epoxy hydrolase for obtaining L-tartaric acid which is precipitated with an equimolar amount of CaCl2 in a 35% solution in order to obtain calcium tartrate crystallisation,
the obtained crystals are thereafter submitted to a grinding process in a mortar in order to obtain the specific calcium tartrate composition according to the invention.
Calcium tartrate crystals can also be obtained in a almost 99% pure calcium tartrate.4H2O by using tartaric acid obtained from natural sources such as raisin or tamarind (see FR 2109317, EP 0919935A1, GB 1183449, U.S. Pat. No. 3,957,579, incorporated herein by reference).
Another aspect of the present invention is related to a plaster composition comprising the calcium tartrate retarding composition according to the invention, possibly combined with other additives already used in plaster compositions, preferably one or more additives selected from the group consisting of citric acid, acrylic latex, aliphatic alcohol, magnesium sulfate, potassium sulfate, cellulose ether, malic acid, adipic acid, several tartaric acids such as L(+)-tartaric acid, D,L-tartaric acid and meso-tartaric acid, salts of said tartaric acids such as potassium and sodium tartrates and corresponding bitartrates, protein hydrolysates, monocalcium phosphate, ketogluconic acid, starch ethers, calcium hydroxide and calcium carbonate.
The plaster composition according to the invention can be used in any suitable plaster composition usually used in construction sites by plasterers such as wall plaster (putzgips, maschinenputzgips), plaster boards, ready-to-use plaster, gypsum filler or coat plaster.
Preferably, the specific calcium tartrate retarding composition according to the invention can find applications as a retarder in the setting of any calcium sulfate hemihydrate composition wherein a retarding effect of such composition is needed.
The calcium hydrate used in said plaster can be obtained from any source such as mining or from fly ash. Said composition can be a xcex2-calcium sulfate hemihydrate form or any other crystalline form well known by the person skilled in the art (xcex1 form, anhydride I, II or III, etc.).
The calcium tartrate retarding composition according to the invention can also find other applications in the retarding of the setting for other related products presented in a powder form such as cement (that find also applications on construction sites for the preparation of mortar, concrete constructions, etc.).
Preferred compositions of the calcium tartrate retarding composition according to the invention are described in the following examples.
A preferred particle size distribution is described in the Figures and in Table 1, which are presented as preferred and non-limiting execution examples of the present invention.